The Avengers sets out to be the ultimate superhero movie, the biggest, smartest, most engaging blockbuster of them all. Will the 'more is more' strategy succeed? And why is this big-budget epic really a film about broken people?

Do we need another hero? What about six? And is there room for all of them on screen at once? Marvel Studios has been betting, for some years, that this is exactly what we want - overload. They've been introducing a succession of blockbuster movies starring characters from the Marvel Comics pantheon that have been designed to lead up to the Big Kahuna of 2012: The Avengers, the ultimate superhero mash-up in which a dirty half-dozen of those characters, blessed and cursed with gifts and super-powers, join forces to save the world.

The cast includes Robert Downey jr, Mark Ruffalo and Scarlett Johansson; the characters include Iron Man, the Hulk and Black Widow. And there's an additional, tantalising element: it is written and directed by Joss Whedon, creator of the Buffy the Vampire Slayer series, a man who is not only an unashamed fanboy but also one of the smartest, subtlest and most inventive writers working in popular culture today.

The movie opens in New Zealand on April 25, a week ahead of its US release (in Britain, it's called Avengers Assemble to avoid confusion with the British TV series that gave us the stylish adventures of John Steed and Mrs Peel). Details of the plot and many of its characters have been kept under wraps, although in the past few weeks Marvel has been releasing trailers that fans have examined and decoded exhaustively.

There has been a flurry of rumours, including an impossible one, emanating from an unwitting cast member, about the presence of Spider-Man (a Marvel creation licensed to another studio).

But Whedon, while cagey about specific plot details, is not a man to keep his ideas to himself. He's a fastidious, playful speaker, with a distinctive turn of phrase and way of talking - saying something such as, "I've been thwarted innumerable times", he sounds like a geeky John Gielgud. And he is fluent, entertaining and expansive about the kind of movie he wants to make - the film, he says, he has been waiting his whole life to create. I had the chance to talk to him before and after the shoot about his plans and how they have been realised. On the first occasion, in August 2010, he had come to Australia as a keynote speaker at several writers' festivals; right now, he's back in Los Angeles, preparing for the moment when the ultimate superhero adventure meets its audience.

He always knew, he told me, while he was still working on the script in 2010, the kind of film he didn't want to make. "I've felt that superhero movies were a little too nihilistic or a little too clean. Or both. I felt like I really want to show the idea of being a hero as something more than powers. And really stick the screws in and make it personal and make it tough.

"So I said to Marvel, let's make a movie that feels old-fashioned in its concept of heroism but modern in its sense of the realities of the cost of battle."

And that is exactly what he has been able to achieve, he says, about 18 months later. "This is why I love Marvel. Everything that I said to you before still applies. Other studios would panic. They'd go, 'Wait, maybe it's a horror movie, maybe it's a romantic comedy, why does Thor have a cape? I'm scared'."

There are ways in which Whedon, 47, is an obvious choice for the Avengers project. He is a gifted storyteller with a long track record. He is steeped in the Marvel back catalogue. He worked on the Captain America: The First Avenger script. He wrote early instalments of the Astonishing X-Men comic-book series. He was in the frame, for some time, to direct a new version of DC Comics' Wonder Woman for Warner Bros.

But although his talents stretch to directing, writing and producing, although he's worked on screenplays, TV series, internet musicals and comic books, and has created some innovative television shows - above all, the deservedly legendary Buffy - he has directed only one other feature film. This is Serenity (2005), a movie that grew out of Firefly, a sadly short-lived sci-fi series he created.

Serenity, although it had a relatively small budget and was born out of the frustration of having had a series cancelled, had at least one thing in common with The Avengers, Whedon notes - a storytelling challenge. How do you make a film about an ensemble cast with a long history and backstory that can work for aficionados and newcomers alike?

In The Avengers, the members of the superhero posse have extensive Marvel backstories and most of them have been re-established in recent movies written and directed by others. These films have contained teasers for the future movies. Whedon has inherited a set-up, to which he is expected to bring his own creative ingenuity. But he also had to make a film for people who have no idea why Bruce Banner, aka the Hulk, is not a man to tease during a crisis.

"We're not a team, we're a time bomb!" he cries in a line from the trailer. They are pitted against a manipulator and that's the source of his strength. Amid speculation about the sources of evil, the one villain we know about is Loki, the Norse trickster god played by Tom Hiddleston. "Loki's not stupid," Whedon says. "He's an observant fellow. He knows that the Avengers is a terrible idea for a superhero team. They really don't belong in the same movie, let alone in the same room. And he's not afraid to push that a little bit."

It makes sense to think about the Avengers' weak points, Whedon says. "I do have Earth's mightiest heroes. And if I can't destroy them a little bit from the inside, then honestly, they're gonna win. In, like, 20 minutes."

One well-known figure Whedon has been able to make more of is Banner. The character has been not only in the comic books but also in the TV series (with Bill Bixby), as well as a couple of movies: Ang Lee's psychologically tormented Hulk (2003), with Eric Bana; and Louis Leterrier's 2008 The Incredible Hulk, with Edward Norton.

Although the scene was set for Norton to appear in The Aven-gers, he and Marvel parted ways. So Whedon has had the chance to rethink the character. And, he says, "It's been a very happy confluence of events. Mark [Ruffalo], first of all, is an extraordinary actor and working with him has been extremely fun. We had a lot of ideas, but we both had the same basic concept, which is that we want the Banner from the TV show, the one who is busy helping everybody else, as opposed to the one who is completely obsessed with his own problems." In the two previous movies, Whedon says, "Bruce spends all his time in agony over himself. We liked the idea that he's just trying to be a good person, to stay out of sight and make himself useful in the world.

"Mark is so open to the audience, you relate to him so much, and he's so talented. He brings a lot of humour to the role and a little danger. He's not a wimpy Banner. He doesn't look like a guy who couldn't hurt a fly; he looks like he's decided not to hurt a fly."

Lee's Hulk had its merits, but it was badly hamstrung by the unconvincing special effects that made the angry green giant look faintly ludicrous. Now, Whedon says, "The tech exists as it never did before and all the work we did designed the Hulk to look like Mark. And for the first time ever, and that includes the comic books, this is the same guy". Banner and his Hulk manifestation finally look alike.

In the past, Whedon has been known for what he hasn't made: for unrealised projects, frustrations, unmade scripts. But things are a little different in 2012. Straight after finishing The Avengers, Whedon shot, on impulse, a low-budget modern version of Shakespeare's Much Ado about Nothing, over 12 days, in a single location, using many actors who'd worked with him on his TV projects. It's due for release later this year.

Then there is another feature, In Your Eyes, that's in production now; it is, he says, "A little film that I wrote many years ago, a slightly paranormal romance that's very much a people-talking-about-their-feelings movie. It's very sweet and I had a great time with it".

There's the documentary, Comic-Con Episode IV: A Fan's Hope, an affectionate tribute to hardcore sci-fi and fantasy fandom that he has produced and appears in. And there's also The Cabin in the Woods, a script intended as a "loving hate letter to the horror movie" that was filmed in 2009 and had its release delayed by financial difficulties at MGM. It will finally be in US theatres on April 13.

Meanwhile, Whedon is waiting for the release of The Avengers, and the critical and box-office verdict. Having presided over a $US220 million budget, he's well aware of the pressure of expectation. "I have a sense of it; I care," he says. "I always enter into any job with the covenant that you deserve to make your money back. But I don't worry about it. It doesn't affect my career, it doesn't affect whether or not I am going to make another giant movie. I feel like there's a place for me in a lot of different areas and there are so many different stories I want to tell and in so many different ways."

Going into the project, he says, he thought to himself, "This movie has to be great, or I'll be sad all the time". But having a massive budget didn't send him into ecstasies of excess. "I didn't spend a lot of time going, 'I can make this bigger'. But there is a lot of flying around in this movie. And that's exciting for me.

"I had to write a treatment of the final battle before I wrote the script, because people had to start work. Things had to move. And the treatment I wrote had five acts and a prologue, just for the battle. We all said ... we're going to have to trim this down. But by god, we filmed it all." Yet, he says, "You're not going to feel pummelled. We try to give it texture and rhythm. But these guys, they go out with a bang".

At the same time, he is aware that "special effects can destroy a movie. An unlimited budget can destroy a movie". In fact, he adds, "You could say to me, 'You've got the Avengers, but all they can do is sit in a room and talk'. And with those actors I'd still have the time of my life".